Referring to FIG. 1, a conventional water wave-type power generating device includes a base 11 for mounting a plurality of power generators (not shown), and a plurality of driving units 13 disposed on the base 11 at left and right sides thereof. Each driving unit 13 has a connecting seat 131 secured on the base 11, a rotary member 132 rotatably mounted on the connecting seat 131 and having a plurality of blades 134 rotated by water waves, and a transmitting member 133 coupled with the rotary member 132 and the respective power generator. The transmitting member 133 is disposed on the connecting seat 131 and includes two bevel gears 135, 136 to transmit a rotational drive from the rotary member 132 to the power generator so as to generate electric power. Since only a part of wave energy is received by the rotary member 132 to generate the rotational drive to the power generator, the power generating efficiency is limited. Moreover, the driving units 13 are subjected to impact of the water waves, which may result in damage to the driving units 13 after a long term use.